Abstract Kaposi's Sarcoma (KS) is the most common AIDS-defining cancer. Combined anti-retroviral therapy (cART) has greatly reduced KS-associated mortality among AIDS patients; however, a serous clinical problem exists in that up to 50% of HIV+KS+ patients in the United States and 61% in Sub-Saharan Africa never achieve complete remission even with chemotherapy and a reduction in HIV viral loads. No definitive study has identified if there are tumor-associated features or mechanisms are associated with cART-progressive KS (progressors) vs. the responder (responders) phenotype. Our study utilizes a robust pre-cART KS tissue collection (n = 224) from the ACSR that originated from the Antiretrovirals for Kaposi's Sarcoma (ARKS) clinical trial. 36% of ARKS participants experienced continued progression of KS despite a reduction in viral loads and restored T-cell counts, in contrast to the rest of the cohort who experienced a reduction or elimination of tumors by the 1-year trial endpoint. This application poses several hypotheses concerning the progressor phenotype: 1) prior to the initiation of cART, an increase in HIV and/or KSHV-infected immune cells enhances a stimulatory immunological profile in the progressor phenotype, 2) there are significant differences in the upregulation of KSHV lytic and immunomodulation genes, which stimulate increased immune cell recruitment into the pre-cART tumor microenvironment and promote tumor progression through cytokine-signaling inflammatory processes enhanced in part by the cART-revitalized immune system, 3) the highly inflammatory nature of KS tumors, comprising a complex mixture of immune cells with which to interact, selects for unique and/or elusive HIV genotypes, distinct from plasma HIV, that promote stimulatory processes to continue in the progressor phenotype. Our Specific Aims address these hypotheses using a combination of advanced immunohistochemistry, virus specific cellular localization (DNA and RNAscope), high-throughput gene expression analysis (RNAseq), and a novel deep sequencing approach applied to HIV. Innovations include the unprecedented amount pre-cART KS tumor material available for the study, advanced imagining technologies, in vivo KS tumors gene expression studies, the use of the newer PacBio SMRT sequencing approach applied to HIV, and machine-learning approaches that can define non-linear associations in complex data sets. This project will define the largest well-characterized set of combinatorial features related to cART-associated KS outcomes derived directly from KS-associated biomaterial. Identifying clinically relevant immune factors in the tumor niche pre-cART will pave the way for the development of future mechanistic studies on the functions of both viral and cellular genes that are involved in cART resistant tumor progression.